This invention relates to a fuel cell and especially to improvement of durability to the heat cycle of a molten carbonate fuel cell.
Structure of the conventional fuel cell shown in FIG. 2 is explained below.
In FIG. 2, cathode 3 and anode 2 are placed above and under electrolyte plate 1 to interpose the electrolyte plate 1 therebetween, forming a unit cell. The unit cells are laminated via separator plates 4 having fuel flow grooves 5 which supply fuel gas to anode 2 and oxidizer flow grooves 6 which supply an oxidizer gas to cathode 3. The conventional electrolyte plate is made of porous ceramic plates of lithium aluminate powder (LiAlO.sub.2 powder) reinforced with alumina fibers, etc.
As electrode material, a sintered Ni porous material has been used for anode and a NiO porous material for cathode.
As materials for separator plates, there have been used high temperature corrosion resistant materials such as austenite stainless steels, for example, SUS316, SUS304, SUS310, etc.
As electrolytes, there have been used mixtures of carbonates such as Li.sub.2 CO.sub.3, K.sub.2 CO.sub.3, Na.sub.2 CO.sub.3, etc., which are impregnated in the electrolyte plate. The electrolytes are solid at room temperature and melt at higher than about 490.degree. C. and flow to the interfacial boundary of the electrode and the separator groove to bring about a chemical reaction to generate power.
On the side of anode 2, a reducing reaction with hydrogen takes place and on the side of cathode 3, an oxidation reaction takes place with oxygen in air in an alkali atmosphere. Therefore, a corrosion resistant austenite steel is used for the separator plate.
The unit cells are laminated in a compressed state under a certain load to increase reactivity and to prevent leakage of gas to outside.
Shown FIG. 3(A) as characteristics during operation of the cell, after temperature is elevated, a fuel and an oxidizing gas are introduced to generate power. After completion of power generation, temperature is gradually reduced to room temperature.
In the future, the fuel cell is promising as a substitute for thermal power plants and further, it is expected that nuclear power plants will be used for base load and fuel cells will be used for daily load.
Therefore, the fuel cell is required to be excellent in heat cycle property so that it can withstand daily load operation (DSS operation).
FIG. 3(B) shows changes in elongation of the parts in a fuel cell at starting-up and stopping. With elevation of temperature during starting-up, elongation of the separator plate and electrolyte plate of the cell occurs as shown by .delta..sub.S and .delta..sub.E, respectively. The elongation .delta..sub.S of the separator plate is larger than the elongation .delta..sub.E of the electrolyte plate. This is because the linear expansion coefficient of the separator plate is greater than that of the electrolyte plate as shown in FIG. 4.
FIG. 5(A) is a plan view and FIG. 5(B) is a sectional view, wherein the arrows indicate the direction of elongation of a cell body. As can be seen from these figures, with an increase in cell temperature, elongation occurs from the center in four directions. The elongation .delta..sub.S of the separator plate 4 is greater than the elongation .delta..sub.E of the electrolyte plate and since the cell undergoes compression load F, electrolyte plate 1 tends to elongate owing to the elongation .delta..sub.S of separator plate 4. Therefore, electrolyte plate 1 which is lower than separator plate 4 in tensile strength undergoes repeated tension and compression at every starting and stopping of the cell (elevation and reduction of temperature).
Japanese Patent KOKAI (Laid-Open) No. 71564/83 discloses a conventional method of preventing the electrolyte plate from cracking due to the heat cycle, but this method gives no consideration to matching the expansion coefficient of the electrolyte plate and that of the separator plate.
Generally, ceramic materials are high in strength against compression, but are low in tensile strength and hence the problem is fatigue strength of the electrolyte plate at a heat cycle.
When cracking of the electrolyte plate occurs due to the fatigue, hydrogen on the anode side mixes with air on the cathode side to bring about an oxidation reaction to produce water. As a result, heat is generated simultaneously with considerable reduction of power generation reaction, resulting in problems of corrosion of the separator plate or electrode to cause a reduction of the lifetime of the cell body.